This invention relates to enhancement in the efficiency of a laser device and reduction in the cost thereof.
FIG. 1 is a schematic view which shows a prior-art axial-flow type laser device disclosed in the official gazette of Japanese Patent Application Laid-open No. 59-125682 by way of example.
Referring to the figure, numerals 101-104 designate electric discharge tubes, which have outlets for a gas 105, 108, 109 and 112 and inlets for the gas 106, 107, 110 and 111, respectively. Numeral 113 designates a Roots pump, numeral 114 a rotary pump, and numeral 115 a gas feeder. Numerals 116 and 117 indicate valves. Shown at numeral 118 is a power source. Reflectors 119 and 120 constitute a resonator. The Roots pump 113 has a discharge port 121, and the electric discharge tubes 101-104 are respectively provided with anodes 122-125 and cathodes 126-129. Ballast resistors 130-133 are connected to the respective anodes 122-125. In the figure, arrows indicate the flow directions of the gas.
In the axial-flow type laser device as stated above, the four discharge tubes 101-104 are connected in optical series for the purpose of attaining a high laser output, and a high voltage is applied across the respective anodes 122-125 and the corresponding cathodes 126-129 of these discharge tubes 101-104 through the ballast resistors 130-133 by the power source 118. The resonator of this laser is constructed of the reflectors 119 and 120. In addition, the Roots pump (also called "mechanical booster pump") 113 and the rotary vacuum pump (hereinbelow, written as "rotary pump") 114 as an auxiliary pump for the Roots pump 113 are employed as blower means for causing the gas from the gas feeder 115 to flow at high speed.
In the operation of the prior-art laser device when the high voltage of about 30 kV is supplied from the power source 118, glow discharges develop across the anodes 122-125 and the corresponding cathodes 126-129 and excite the laser medium within the discharge tubes 101-104. A laser beam is caused to resonate by the reflectors 119 and 120. The mass flow of the gas must then be increased to prevent the laser output from being saturated by a raised gas temperature ascribable to the electric discharges and allow the device to be made compact.
In general, to produce a single (or low-order) mode of good beam-condensing performance, high-order modes must be suppressed by an aperture whose diameter is 10 mm or so. The provision of the aperture, however, decreases the laser output sharply. Therefore, commonly the electric discharge tubes are sized with an inside diameter of about 10 mm to serve as the aperture. Heretofore, to increase the mass flow within such fine discharge tubes, a gas stream not slower than 100 m/sec has been forcibly obtained with the Roots pump 113.
FIGS. 2(a) and 2(b), FIGS. 3(a) and 3(b), and FIGS. 4(a) and 4(b) show examples of the schematic sectional views of the prior-art laser device and beam modes 138 they produce. In these figures, numeral 139 indicates the beam waist of the single (or low-order) mode. The axis of ordinates r of the beam mode 138 represents a distance in the radial direction of the discharge tube, while the abscissas along axis I represent a laser beam intensity. FIGS. 2(a) and 2(b) exemplify a case where the laser is oscillated with a large-diameter discharge tube. In this case, the beam produced exhibits a high lasing efficiency, as illustrated by a straight line II in FIG. 5, but it is of high-order mode of inferior beam-condensing performance as seen from the beam mode 138 in FIG. 2(b).
In the example of FIG. 3(a), a conventional aperture 140 is inserted in the resonator. In this case, the beam mode 138 in FIG. 3(b) becomes the single mode, but the lasing efficiency is extremely low as illustrated by a straight line III in FIG. 5. The low efficiency is attributed to the fact that the gain of a vertical line part in FIG. 3(a) is not utilized at all. FIG. 4(a) shows the prior-art example in the case where the discharge tube also serves as the aperture. With this example, both the beam mode 138, illustrated in FIG. 4(b), and the lasing efficiency, indicated by a straight line IV in FIG. 5, are superior. However, as stated before, a gas stream of high speed must be maintained by the Roots pump to allow the resonator to be made compact.
As an example, FIG. 6(a) is a schematic sectional view showing a prior-art laser device disclosed in the specification of Japanese Patent Application No. 58-69532, and FIG. 6(b) is a diagram showing the intensity distribution of the emergent laser beam in the radial direction. Numeral 1 designates a partial reflection mirror, on the surface of which a thin film or partial reflection film 12 is formed. Numeral 2 designates a total reflection mirror. Numeral 3 indicates a laser medium, for example, a gas excited by electric discharge in a CO.sub.2 laser or a glass excited by a flash lamp in a YAG laser. A laser beam 4 is generated in an optical resonator constructed of the mirrors 1 and 2, and the emergent laser beam 41 is taken out. The intensity distribution curve of the emergent laser beam 41 in the radial direction is shown as curve 5. An aperture 6 is made of a laser beam absorber, and has an opening at its central part.
In operation, a laser beam reciprocating between the partial reflector 1 and the total reflector 2, which together form the optical resonator, is amplified by the laser medium 3 and becomes the laser beam 4.
Since the laser beam reaching the outer peripheral surface of the aperture 6 is absorbed, the laser beam 4 has its contour defined and its transverse mode (the intensity distribution of the laser beam in the radial direction) limited to a Gaussian form (a normal distribution form). The thin film 12 is formed on the surface of the mirror 1, and part of the laser beam 4 is externally derived as the emergent laser beam 41. The curve 5 shown in FIG. 6(b) illustrates the sectional profile of the intensity distribution of the derived laser beam.
The Gaussian laser beam has good beam-condensing performance, and is deemed a laser beam of good quality. Most laser devices commercially available generate Gaussian laser beams.
The effect of the aperture 6 will be explained. Among laser beam modes, the one of the smallest contour is of the Gaussian form. Therefore, when the aperture 6, having a diameter through which the laser beam in the Gaussian form barely passes, is inserted in the optical resonator, the laser beam outside the Gaussian form is cropped by aperture 6 during the reciprocation in the optical resonator. The portion of the dropped laser beam is absorbed and greatly attenuated by the aperture with the result that only the laser beam in the Gaussian form remains.
It is known from experiment that the ultimate aperture diameter .phi..sub.a has the relation of .phi..sub.a =3.2.omega. to 3.4.omega., where .omega. is the radius at the point at which the intensity of the Gaussian laser beam becomes 1/e.sup.2 of the intensity at its center.
The prior-art laser device constructed as described above has had the problem that the laser beam of the highest attainable quality is limited to one having the Gaussian mode.
In addition, the prior-art laser device has produced the Gaussian laser beam in such a way that the contour of the laser beam is regulated by inserting the aperture as stated above. The oscillating efficiency of a laser becomes highest when a laser beam and a laser medium have equal sizes i.e., when no aperture is employed. Since, laser media including gaseous laser media excited by electric discharge for use in the CO.sub.2 laser and the glassy laser medium excited by the flash lamp for use in the YAG laser are usually nonhomogeneous, it has been common practice to make the excitation space somewhat larger than the laser beam and limit the size of the beam by an aperture. Thus the emerging uniform beam derived from only the good quality part of the laser beam. Therefore, the oscillating efficiency of the laser has been limited.
Further, the partial reflection mirror 1 absorbs a portion of the laser beam 4. Because the laser beam power is centrally concentrated, the central part of the partial reflector 1 is intensely heated by the absorbed laser beam and is thermally deformed.
Moreover, because the axial flow type laser device in the prior art is constructed as explained before, it has incurred the following problems:
(1) The diameter of the discharge tube cannot be made large, and the high-speed gas stream is required for attaining a great mass flow. PA1 (2) The Roots pump 113 is necessary for achieving the requirement (1), and contamination of the interior of the oscillator with gear oil etc. is problematic. PA1 (3) In order to reduce the pressure loss of the gas stream system, the discharge tube must be divided, so that the laser device becomes structurally complicated.
FIG. 7 is a sectional constructional view which shows a prior-art laser device disclosed in the specification of Japanese Patent Application No. 58-4222. Referring to the figure, the laser device has a pair of reflectors, 1', and 2', of which the former 1' is a concave spherical mirror and the latter, 2', is a convex spherical mirror. Numeral 3' designates a laser medium and numeral 4' a pierced reflector, which is a beam deriving mirror. A ring-shaped laser beam 5' is externally derived, and an aperture 6' regulates the contour of the laser beam. Numeral 61' indicates a laser beam absorber, and numeral 52' a laser beam falling on the laser beam absorber 61'. Shown at numeral 10' is a base.
Next, the operation of the laser device will be explained.
The pair of reflectors 1' and 2' are opposingly arranged with the laser medium 3' held therebetween to form a confocal unstable type optical resonator. Light reciprocating between both the mirrors is amplified by the laser medium 3' and is gradually shifted from the axis of the optical resonator, whereupon it is externally derived as the collimated laser beam 5'.
The laser beam 5' to be derived has its contour adjusted by passing through the aperture 6' placed within the optical resonator. The aperture 6' is provided with the laser beam absorber 61', by which the laser beam 52', to be cut down to adjust the contour of the laser beam, is absorbed.
In a prior-art laser device constructed as above, when it is intended to obtain a laser beam of high quality having an adjusted contour, part of the laser beam must be absorbed by the laser beam absorber, lowering in the efficiency of laser output.